1. Field of the Invention
The present invention generally relates to an optical information storage unit, and particularly relates to an optical information storage unit for reproducing information recorded on lands and grooves of a magneto-optical recording medium by extracting 0th order light and +/xe2x88x921st order lights by means of diffraction grating from a reflected beam.
The present invention further relates to an optical information storage unit which has a magneto-optic detection optical system provided with phase compensation means which supplies different amounts of phase compensation for land reproduction and for groove reproduction, in order to realize a high-density magneto-optical recording medium.
In this specification, an xe2x80x9cinformation storage unitxe2x80x9d refers to an apparatus which records information on and/or reproduces information from a recording medium.
2. Description of the Related Art
Presently, an optical recording medium is widely used as a recording medium from which audio and video signals can be reproduced. Particularly, considerable research and development have been carried out for a magneto-optical recording medium and a phase change recording medium for use as a rewritable high-density recording medium. A recording density of an optical recording medium on which information is stored in a spiral or concentric manner can be improved by reducing a track pitch and/or by increasing a line recording density.
A reduction in track pitch and an increase in line recording density can both be achieved by introducing a short wavelength semiconductor laser for storage and reproduction. However, a semiconductor laser of a short wavelength, i.e., in the range of green or blue, is not yet on the market at a reasonable price since it is not stable in room temperature and thus is not capable of continuous oscillation for a long period of time.
Thus, there is a need for a method which can improve a recording density of a magneto-optical storage unit while using a laser of a presently used wavelength. One such method is as an MSR (magnetic super resolution) method described below.
With a RAM medium such as a magneto-optical recording medium, light beams having equal wavelengths for writing and reading are utilized, whereas with a ROM medium having prerecorded information, phase pits are formed by means of a gas laser having a short wavelength.
The RAM medium and the ROM medium have substantially identical reproduction conditions. However, the RAM medium has a disadvantage in that a short-wavelenght light source not yet on the market is needed for high-density recording of information.
A land-and-groove recording method is a very important method used in the development of a high-density storage medium, since a recording density can be doubled with the same line storage density and the same track pitch.
It is reported that with the magneto-optical recording medium, the above-mentioned MSR method not only can improve the line recording density but also reduces an intertrack crosstalk. Therefore, various attempts have been made to apply the MSR method to the land-and-groove recording method. However, conditions for generating magnetic super resolution are extremely complicated. For example, such conditions may be that a reproduction laser power depends on a line density, a reproduction magnetic field is necessary and at least three magnetic layers are necessary. Therefore, the MSR method tends to be unstable and costly.
Also, Japanese Laid-Open Patent Application No. 8-7357 proposes an optical information storage unit which can reduce crosstalk originating from lands or grooves by appropriately choosing the depth of the grooves.
Further, Japanese Laid-Open Patent Application No. 9-128825 proposes an optical information storage unit which records and reproduces information in/from lands having substantially the same width as that of grooves having an optical depth of substantially xe2x85x9 wavelength (57 nm).
However, in an actual optical system, absolute values of a phase difference necessary for land reproduction and a phase difference necessary for groove reproduction do not always coincide. This is because it is a component such as a right-angle mirror or a polarization beam splitter of an optical information storage unit that causes a phase difference.
Therefore, as described in Japanese Laid-Open Patent Application No. 9-272868, the present applicant proposes an optical information-storage unit provided with a wave plate and a diffraction grating so as to obtain a stable phase difference by appropriately adjusting an inclination of the wave plate.
In the optical information storage unit of the related art, if the track pitch is narrowed, crosstalk may occur during reproduction. The crosstalk is caused when data signals of neighboring regions are mixed into an output signal of the region to be reproduced. With a land recording or a groove recording of the related art, since each land is provided between grooves or each groove is provided between lands, there is a certain separation between regions in which information is written. Therefore, possible crosstalk is suppressed. However, with a land-and-groove recording, since information recording regions are adjacent to each other, a reproduction feature is likely to be influenced by the crosstalk.
Japanese Laid-Open Patent Application No. 8-7357 describes reducing the crosstalk from lands or grooves by appropriately choosing the depth of the grooves. However, since an ordinary medium will be free of crosstalk when the groove depth is approximately ⅙ wavelength, a carrier level of the signals will be reduced with the magneto-optical recording medium compared to an ordinary case where the groove depth is xe2x85x9 wavelength. Also, push-pull signals, which serve as track error signals, are reduced. Further, it is reported that the abovedescribed crosstalk-free conditions are easily altered as a result of change in a Kerr-ellipticity ratio, focal error of an objective lens, and spherical aberration.
Japanese Laid-Open Patent Application No. 9-128825 discloses an optical information storage unit in which information is recorded and reproduced in/from lands having substantially the same width as grooves having an optical depth of substantially xe2x85x9 wavelength (57 nm). However, since two read channels are required for land reproduction and groove reproduction, there is a problem that an optical system has a complicated structure and thus is expensive.
Further, Japanese Laid-Open Patent Application No. 9-272868 discloses a structure in which a stable phase difference is obtained by regulating merely a relationship between a diffraction grating and a wave plate, and thus an adjustment of the phase difference is not possible.
Accordingly, it is a general object of the present invention to provide an optical information storage unit which can satisfy the needs described above.
It is another and more specific object of the present invention to provide an optical information storage unit which can be adjusted to a stable phase difference.
In order to achieve the above objects according to the present invention, an optical information storage unit includes a first wave plate on which reflected light from a magneto-optical recording medium is incident; a diffraction grating on which the light having been transmitted through the first wave plate is incident; a second wave plate on which the light having been transmitted through the diffraction grating is incident; and a polarization detecting unit on which the light having been transmitted through the second wave plate is incident. Also, the optical information storage unit may further include a land-information-signal reproducing-unit for reproducing information formed on lands using signals output from said polarization detecting unit, and a groove-information-signal reproducing-unit for reproducing information formed on grooves using signals output from said polarization detecting unit.
With the optical information storage unit described above, optimum reproduction signals can be obtained by adjusting an inclination of the first wave plate and the diffraction grating.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.